Paralogania klubovi, Märss & Karatajūtē-Talimaa, 2002

Märss, Tiiu & Karatajūtē-Talimaa, Valentina, 2002, Ordovician and Lower Silurian thelodonts from Severnaya Zemlya Archipelago (Russia), Geodiversitas 24 (2), pp. 381-404 : 393-395

Paralogania klubovi n. sp. ( Figs 8-10 )

HOLOTYPE. — LIG 35-834, trunk scale ( Fig. 8G ).

ETYMOLOGY. — In honour of Dr. B. A. Klubov (Magadan, Russia), researcher of Silurian and Devonian deposits of the Pioneer Island in 1976.

MATERIAL EXAMINED. — 32 scales from one sample.

TYPE LOCALITY AND HORIZON. — Pioneer Island, Member 2, bed 1, sample 2zh, Lower Silurian, middle(?) Llandovery.

OCCURRENCE. — A single sample 2zh from the Pioneer Island (Member 2, bed 1, see Klubov et al. 1980). Based on revised biostratigraphy of conodonts from this member by P. Männik, this level is dated as middle(?) Llandovery, Early Silurian.

DIAGNOSIS. — Scales very small (0.30-0.45 mm). Head scales low, flat, with deeply crenulated margins around the crown. Transitional scales elongated with low base occurring as a narrow weak wall around the large, shallow pulp depression. Short medial crown area developed only anteriorly. Trunk scales with narrow and deeply furrowed, or with wide and shallow medial area. Wider lateral crown areas not developed in all trunk scales. Lateral spinelets (two to three) situated anteriorly, and may occur as flat widened to flat short areas. Base low, anteriorwardly more convex. Numerous openings of dentine canals situated on the surface of pulp depression of the head and transitional scales. Short pulp canal developed only in trunk scales. Dentine canals comparatively long, branching at several levels with widened proximal parts.

DESCRIPTION

Morphology

Very restricted, insufficient material does not allow a complete morphological description of a set of scales. Nevertheless, the basic morphological types, head, transitional and trunk scales are represented in the collection.

Head scales ( Figs 8A, B ; 9A, B ) have a low and flat shape which is characteristic for early loganiids. The crown surface is smooth. The crown margins are rounded and deeply crenulated, the cuts being wide. The distal crown area is distinct in more elongated scales ( Fig. 8B ). The base is low and smaller than the crown ( Fig. 9A, B ), with its central part of the base containing openings of dentine canals.

A single scale, figured in Fig. 8C , is ascribed to the transitional scale type. Its crown surface is flat and smooth. The short medial area is distinct anteriorly, and laterally the cuts are not as deep as in head scales. The distal part is monolithic with a tapered posterior point. The base is low and in the form of a narrow wall seen along all the perimeter. The pulp depression is shallow and very large; dentine canal openings are placed evenly.

Several morphological varieties are observed among trunk scales.

Type A. The crown is composed of a narrow central (medial) area, and of one pair of lateral ones ( Fig. 8 D-E). The central area is higher than the remaining crown surface and separated from lateral areas by deep longitudinal grooves. A comparatively deep groove (or furrow) is situated at the proximal part of the central area only ( Fig. 8D ) or is extended over its length ( Fig. 8E ). Wing-like lateral ribs, two to three on each crown side, correspond to the lateral thornlets of trunk scales of Paralogania . The base is low and flat, with openings of dentine canals ( Fig. 9C ); it can also be anteriorly slightly convex.

Type B. Elongated scales with a rather narrow, elevated central area that tapers posteriorly. A shallow furrow is situated along the central area ( Figs 8F ; 9 D ). Lateral crown areas repeat the outline of the central one. They form lateral rib, one on both sides.

Type C. Mostly typical for trunk scales of Paralogania ( Fig. 8G , selected for the holotype). It illustrates the early stage of formation of rows of thornlets, characteristic for this genus. The crown is composed of an elongated rhomboidal central area and one pair of lateral ribs. The proximal part of the lateral ribs is divided into two thornlets, i.e. the disintegration of a complete lateral rib into a row of separate thornlets has taken place.

Type D. It represents a morphological variety of scales which is almost entirely composed of rhomboidal and elongated central areas. The lateral areas are very narrow ( Fig. 8H ) or are not developed at all ( Fig. 8I ). The lateral crown walls are smooth.

Scales of C and D types have bases that are comparatively high and more convex anteriorly. The pulp canal starts from the depression in the distal corner of the base ( Fig. 9D ).

Histology

The microstructure of the scales was successfully observed in anise oil. No thin section was made because of insufficient material. Fig. 10 shows the arrangement and shape of dentine canals in the head scales ( Fig. 10A ) and trunk scales ( Fig. 10B ). The proximal part of the dentine canals is rather wide; that is why their openings on the surface of the pulp depression are easily seen even at low magnification. The dentine canals are comparatively long (at least in lateral crown parts), and branch at several levels. The trunk scales of the type A ( Figs 8D ; 9B ) have short pulp canal in its initial stage of development only. Trunk scales with more elongated distal part of the central crown area (types B, C and D) have more distinctly developed pulp canal.

COMPARISON

By the presence of a single pair of lateral ribs (wing-like lateral ledges or thornlets), Paralogania klubovi n. sp. may be attributed to the P. kummerowi (Gross, 1967) - P. martinssoni (Gross, 1967) group (see Karatajūtē- Talimaa 1997: fig. 7g). The head and transitional scales of P. klubovi n. sp. differ from the representatives of the group in their characteristic crown shape and presence of comparatively large openings of dentine canals. Partial disintegration of complete lateral rib into separate thornlets is observed in trunk scales of P. klubovi n. sp. only. In scale size, P. klubovi n. sp. resembles other early species of loganiid thelodonts.

Figures

FIG. 8. — A-I, Paralogania klubovi n. sp.; A, B, head scales; C, transitional scale; D-I, trunk scales; A, B, D-F, H, I, scales in crown view; C, a scale in base view; G, a scale (holotype) in lateral view; A, LIG 35-085; B, LIG 35-086; C, LIG 35-832; D, LIG 35-087; E, LIG 35-833; F, LIG 35-088; G, LIG 35-834; H, LIG 35-835; I, LIG 35-836; J-L, Loganelliidae gen. et sp. indet.; J, scale in crown view; K, posterior part of the same scale; L, enlarged part of left posterior section, LIG 35-089; sample 2zh, Pioneer Island, middle(?) Llandovery, Lower Silurian. Scale bars: 0.1 mm

FIG. 9. — A-D, Paralogania klubovi n. sp.; A, B, head scales in base view (see Fig. 8A, B); C, trunk scale of type A in base view (see Fig. 8D); D, trunk scale of type B in base view (see Fig. 8F); A, LIG 35-085; B, LIG 35-086; C, LIG 35-087; D, LIG 35-088; E, Loganelliidae gen. et sp. indet.; scale in base view (see Fig. 8J, K), LIG 35-089; sample 2zh, Pioneer Island, middle(?) Llandovery, Lower Silurian. Scale bar: 0.1 mm.

FIG. 10. — Paralogania klubovi n. sp., microstructure of the scales; A, head scale, LIG 35-086; B, trunk scale, LIG 35-087; the scales were drawn in anise oil; sample 2zh, Pioneer Island, middle(?) Llandovery, Lower Silurian. Abbreviations: b, base; cr, crown; dt, dentine tubules; odt, openings of dentine tubules; mpc, main pulp canal. Scale bar: 0.1 mm.